Polycarbonate molding compositions are widely used for parts of automobiles and electrical products. They have a good combination of transparency, high impact strength and heat resistance. However, polycarbonate molding compositions do not have a good proccessability during molding process, so other resins are used with the polycarbonate resin. For example, a molding composition comprising a polycarbonate resin and a styrenic resin has good proccessability as well as high notched impact strength.
It is desirable to have a good balance of physical properties such as tensile strength, stiffness, compressive and shear strength as well as impact resistance in the polycarbonate blend compositions. These compositions are used in electric and electronic products such as computer monitor and copy machine housings where the composition is exposed to elevated temperatures, i.e., temperatures above room temperature, for extended time periods. If the composition cannot retain physical properties such as mechanical strength during such exposure, the lifetime of the product will be shortened because of the degradation of physical properties.
Therefore, in addition to absolute physical properties such as tensile strength and impact resistance, it is desirable to have compositions with good long-term heat stability, that is, the ability to maintain mechanical strength during exposure to elevated temperatures. Long-term heat stability is a different physical property from absolute tensile strength that is measured at a particular temperature and at a particular time. Long-term heat stability is a relative value that is measured over a time period. More specifically, long-term heat stability is the time period in which the tensile strength of a composition is reduced to one-half of the initial tensile strength during exposure to elevated temperature. Higher tensile strength does not necessarily correlate with good long-term heat stability. For example, even though resin A has greater tensile strength than resin B, resin B can have better long-term heat stability than resin A.
Furthermore, the polycarbonate molding composition used for housing materials of home appliances and computers should be flame resistant to prevent fires. For this purpose, halogen and/or antimony containing-compounds have been used to render flame retardency to thermoplastic molding compositions. In U.S. Pat. Nos. 4,983,658 and 4,883,835, a halogen-containing compound is used as flame-retardant. The halogen-containing compound, however, results in the corrosion of the mold itself by the hydrogen halide gases released during a molding process and is fatally harmful due to the toxic gases liberated in the case of fire.
U.S. Pat. No. 4,692,488 discloses a thermoplastic molding composition comprising a halogen-free aromatic polycarbonate, a halogen-free thermoplastic copolymer of styrene and acrylonitrile, a halogen-free phosphorus compound, a tetrafluoroethylene polymer and graft polymers of ABS. The use of a phosphorus compound and a perfluoroalkane polymer to render flame retardency to a polycarbonate/ABS resin composition, as disclosed in U.S. Pat. No. 4,692,488, herein incorporated by reference, prevents the dripping of flaming particles during combustion. U.S. Pat. No. 5,292,786, herein incorporated by reference, discloses flame retardant polymer blend compositions comprising a polycarbonate polymer, an ABS resin, a phosphate flame retardant compound and a polymethacrylate. The phosphate flame retardant compounds include monophosphate compounds and diphosphate compounds. Though the resin compositions disclosed in these U.S. patents have satisfactory flame retardency, unsatisfactory surface crack due to the flame retardant migration during a molding process, called "juicing", might occur, thus degrading the physical properties of the resin composition. In order to prevent the juicing phenomenon, an oligomeric phosphate is used in the preparation of a polycarbonate/ABS resin composition, as disclosed in U.S. Pat. No. 5,204,394, herein incorporated by reference. This patent discloses a polymer mixture comprising an aromatic polycarbonate, a styrene-containing copolymer and/or styrene-containing graft copolymer and an oligomeric phosphate or a blend of oligomeric phosphates as flame retardant.
The use of halogen-free phosphate compositions as flame retardants avoids the problems caused by the corrosive by-products of chlorine or bromine-containing flame retardants. However, phosphate compounds have a tendency to decompose and reduce to phosphoric acid and other by-products when maintained at high temperatures such as 80.degree. C. or above for an extended period of time. This phenomenon degrades the mechanical properties of the polycarbonate compositions. As discussed previously, long term heat stability is important in situations where the molded product is exposed to elevated temperatures for extended periods of time.
Zeolites are crystalline aluminosilicates of group IA and group IIA elements such as sodium, potassium, magnesium and calcium. Zeolites are well known and have been amply described in the literature, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., John Wiley & Sons, New York, 1995, pp. 888-925, herein incorporated by reference. Zeolites have been used as adsorbents, industrial catalysts, and water softeners. Zeolites can be modified by ion exchange, however, cation exchange may produce considerable change in various other properties such as thermal stability, adsorption behavior, and catalytic activity (Kirk-Othmer, p. 901).
U.S. Pat. No. 4,464,504 discloses polystyrene and impact resistant polystyrene blend compositions that include zeolites having a pore size exceeding 0.43 nm. The zeolite is said to participate in the reduction of combustibility and smoke reduction of polystyrene. The effect can be further increased by changing the ions on the zeolite by ion exchange. The flame retardants for the compositions of U.S. Pat. No. 4,464,504 are bromine-containing compounds or Sb.sub.2 O.sub.3. There is no disclosure of polycarbonate resins or phosphate flame retardants in this patent.
U.S. Pat. No. 5,149,735 discloses self-extinguishing polymeric compositions that comprise a thermoplastic polymer, an organic aliphatic or aromatic halogen-derivative, an adduct between an inorganic halide and a zeolite, and an organic reactive agent capable of modifying the polymeric matrix. In a Comparative Test, the use of a non-adduct zeolite such as zeolite 4A produced a material with poor self-extinguishment characteristics and was classified 94 V-2. There is no mention of flame retardant phosphate compositions in this patent.
Japanese Patent Application 63-170440 discloses the addition of zeolite containing a specific metal or preferably an organotin maleate to thermally stabilize a styrene-based resin composition containing a bromine based compound as a flame retardant. There is no mention of polycarbonate resins or flame retardant phosphate compositions. Japanese Patent Application 63-92661 discloses a low smoke emitting resin composition that is prepared by combining a halogen-containing polymer with at least one aluminosilicate from the group consisting of clay, mica, and zeolite that has been fired with a zinc compound. There is no disclosure of polycarbonate resins as a resin component or phosphate compositions as flame retardants. The processes described above involve the modification of zeolite by reaction or ion exchange. Therefore, these processes requires more starting materials, resources, and processing steps in the overall manufacturing process. The use of halogen-containing flame retardants or a halogenated polymer would be expected to produce problems with the release of corrosive and toxic materials during high temperature processing or combustion.
Accordingly, in order to overcome the shortcomings mentioned above, the present inventors have developed a new resin composition comprising a base resin comprising a halogen-free polycarbonate, a styrene-containing graft copolymer and a styrene containing copolymer, a flame retardant phosphate composition, and a zeolite.